Merging two spheres with different radii
Bridging two sections with  a different number of vertices will yield untidy topology ... perhaps an approach like this would be more flexible?




Start with some sort of sphere for the lens (here I've used a rounded cube). It helps to set its display property to X-Ray.
Create 2 vertex groups in the lens: one of all the vertices above the equator ('top'), and one of all the vertices below the equator('bottom').
Create 2 high-resolution spheres 'top' (the lower one in the illustration) and 'bottom' (the higher one) to represent the curvatures of your lens. (It helps to set their maximum display to 'wire')
Assign 2 Shrink-Wrap modifiers to the lens. One is set to affect the 'top' vertex group with the target sphere set to 'top'.. the other, vice-versa. Perhaps pop in a Subdivision Surface modifier at the bottom of the stack.
Uniformly scale your target spheres and move them along Z until you are happy. 


Variations might include leaving a wider unaffected margin between the vertex groups, and/or scaling that margin in XY to get the transition you'd like between the spherical sections.